Primary Magenta vs Witch Purple

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Witch Purple is noticeably lighter, more saturated, cooler than Primary Magenta.

Can I use Witch Purple instead of Primary Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Witch Purple is noticeably lighter, more saturated, cooler than Primary Magenta.

Can I use Primary Magenta instead of Witch Purple?

The same difference applies the other way round: Primary Magenta is noticeably darker, more muted, warmer.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.